3

为什么下面两个查询中的第一个查询的执行时间较低，而估计的子树成本较高？

估计的子树成本只是结合其他查询性能指标（如响应时间）来衡量查询计划性能的指南，但当我并排比较两个类似查询时，成本和时间之间存在反比关系让我感到惊讶。

我创建了一个示例数据库来说明这个问题，并为这两个查询包含了生成的 XML 查询计划。我意识到 StateID 作为外键在 County 和 Town 表中的存在是不好的形式，但对我来说这是说明这种现象的最简单方法。

我不只是问“哪个更重要，子树成本或响应时间”。我想了解查询计划子树成本和两个类似查询的响应时间之间如何存在反比关系，以便我可以做出更明智的决策。

请注意，下面的两个查询产生完全相同的结果集。

谢谢！

CREATE TABLE dbo.County (StateID INT, CountyID INT)
CREATE TABLE dbo.Town (StateID INT, CountyID INT, TownID INT)

DECLARE @State TABLE (StateID INT, PRIMARY KEY CLUSTERED (StateID))
DECLARE @County TABLE (CountyID INT, PRIMARY KEY CLUSTERED (CountyID))

INSERT INTO @State
VALUES (27)

INSERT INTO @County
SELECT DISTINCT
    c.CountyID
FROM dbo.County c
JOIN @State s ON c.StateID = s.StateID

-- low time, high cost query
SELECT DISTINCT
    t.CountyID,
    t.TownID
FROM dbo.Town t
JOIN @State s
    ON t.StateID = s.StateID

-- low cost, high time query
SELECT DISTINCT
    t.CountyID,
    t.TownID
FROM dbo.Town t
JOIN @State s
    ON t.StateID = s.StateID
JOIN @County c
    ON t.CountyID = c.CountyID

以下查询计划适用于估计子树成本为 6.06 且经过时间为 1114 毫秒的查询。

<StmtSimple StatementCompId="6" StatementEstRows="273495" StatementId="3" StatementOptmLevel="FULL" StatementSubTreeCost="6.06304" StatementText="SELECT DISTINCT&#xD;&#xA;&#x9;t.CountyID,&#xD;&#xA;&#x9;t.TownID&#xD;&#xA;FROM dbo.Town t&#xD;&#xA;JOIN @State s&#xD;&#xA;&#x9;ON t.StateID = s.StateID&#xD;&#xA;&#xD;&#xA;-- low cost, high time query&#xD;" StatementType="SELECT" QueryHash="0x9347C19165C31DBF" QueryPlanHash="0x98EE57C66D8B347A">
          <StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
          <QueryPlan DegreeOfParallelism="2" MemoryGrant="28144" CachedPlanSize="32" CompileTime="8" CompileCPU="8" CompileMemory="312">
            <RelOp AvgRowSize="15" EstimateCPU="0.388488" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Gather Streams" NodeId="0" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="6.06304">
              <OutputList>
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Parallelism>
                <RelOp AvgRowSize="15" EstimateCPU="3.79725" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Aggregate" NodeId="1" Parallel="true" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="5.67455">
                  <OutputList>
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OutputList>
                  <MemoryFractions Input="0.998723" Output="1" />
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="2" ActualRows="6515" ActualEndOfScans="1" ActualExecutions="1" />
                    <RunTimeCountersPerThread Thread="1" ActualRows="6512" ActualEndOfScans="1" ActualExecutions="1" />
                    <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                  </RunTimeInformation>
                  <Hash>
                    <DefinedTowns />
                    <HashKeysBuild>
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                    </HashKeysBuild>
                    <BuildResidual>
                      <ScalarOperator ScalarString="[Locality].[dbo].[Town].[CountyID] as [t].[CountyID] = [Locality].[dbo].[Town].[CountyID] as [t].[CountyID] AND [Locality].[dbo].[Town].[TownID] as [t].[TownID] = [Locality].[dbo].[Town].[TownID] as [t].[TownID]">
                        <Logical Operation="AND">
                          <ScalarOperator>
                            <Compare CompareOp="IS">
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                </Identifier>
                              </ScalarOperator>
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                </Identifier>
                              </ScalarOperator>
                            </Compare>
                          </ScalarOperator>
                          <ScalarOperator>
                            <Compare CompareOp="IS">
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                </Identifier>
                              </ScalarOperator>
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                </Identifier>
                              </ScalarOperator>
                            </Compare>
                          </ScalarOperator>
                        </Logical>
                      </ScalarOperator>
                    </BuildResidual>
                    <RelOp AvgRowSize="15" EstimateCPU="0.494228" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Repartition Streams" NodeId="2" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="1.8773">
                      <OutputList>
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="2" ActualRows="1017328" ActualEndOfScans="1" ActualExecutions="1" />
                        <RunTimeCountersPerThread Thread="1" ActualRows="1093191" ActualEndOfScans="1" ActualExecutions="1" />
                        <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                      </RunTimeInformation>
                      <Parallelism PartitioningType="Hash">
                        <PartitionColumns>
                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                        </PartitionColumns>
                        <RelOp AvgRowSize="15" EstimateCPU="0.571604" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Inner Join" NodeId="3" Parallel="true" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="1.38307">
                          <OutputList>
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
                            <RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                            <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                          </RunTimeInformation>
                          <NestedLoops Optimized="true">
                            <OuterReferences>
                              <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                            </OuterReferences>
                            <RelOp AvgRowSize="11" EstimateCPU="0.0285019" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Distribute Streams" NodeId="5" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="0.031785">
                              <OutputList>
                                <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                              </OutputList>
                              <RunTimeInformation>
                                <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                              </RunTimeInformation>
                              <Parallelism PartitioningType="RoundRobin">
                                <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="6" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                                  <OutputList>
                                    <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                  </OutputList>
                                  <RunTimeInformation>
                                    <RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                                    <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                                  </RunTimeInformation>
                                  <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                                    <DefinedTowns>
                                      <DefinedTown>
                                        <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                      </DefinedTown>
                                    </DefinedTowns>
                                    <Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
                                  </IndexScan>
                                </RelOp>
                              </Parallelism>
                            </RelOp>
                            <RelOp AvgRowSize="15" EstimateCPU="0.301001" EstimateIO="0.478681" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Index Seek" NodeId="7" Parallel="true" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.779682" TableCardinality="6291320">
                              <OutputList>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                              </OutputList>
                              <RunTimeInformation>
                                <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                                <RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                              </RunTimeInformation>
                              <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
                                <DefinedTowns>
                                  <DefinedTown>
                                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                  </DefinedTown>
                                  <DefinedTown>
                                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                  </DefinedTown>
                                </DefinedTowns>
                                <Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_StateID_CountyID_TownID]" Alias="[t]" IndexKind="NonClustered" />
                                <SeekPredicates>
                                  <SeekPredicateNew>
                                    <SeekKeys>
                                      <Prefix ScanType="EQ">
                                        <RangeColumns>
                                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                                        </RangeColumns>
                                        <RangeExpressions>
                                          <ScalarOperator ScalarString="@State.[StateID] as [s].[StateID]">
                                            <Identifier>
                                              <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                            </Identifier>
                                          </ScalarOperator>
                                        </RangeExpressions>
                                      </Prefix>
                                    </SeekKeys>
                                  </SeekPredicateNew>
                                </SeekPredicates>
                              </IndexScan>
                            </RelOp>
                          </NestedLoops>
                        </RelOp>
                      </Parallelism>
                    </RelOp>
                  </Hash>
                </RelOp>
              </Parallelism>
            </RelOp>
          </QueryPlan>
        </StmtSimple>

以下查询计划适用于估计子树成本为 0.14 且经过时间为 4614 毫秒的查询。

<StmtSimple StatementCompId="7" StatementEstRows="339.324" StatementId="4" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.142839" StatementText="SELECT DISTINCT&#xD;&#xA;&#x9;t.CountyID,&#xD;&#xA;&#x9;t.TownID&#xD;&#xA;FROM dbo.Town t&#xD;&#xA;JOIN @State s&#xD;&#xA;&#x9;ON t.StateID = s.StateID&#xD;&#xA;JOIN @County c&#xD;&#xA;&#x9;ON t.CountyID = c.CountyID&#xD;&#xA;&#xD;" StatementType="SELECT" QueryHash="0xF7B13FA8059B141C" QueryPlanHash="0x31CEE9DAF12E77A5">
          <StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
          <QueryPlan DegreeOfParallelism="1" MemoryGrant="1584" CachedPlanSize="40" CompileTime="5" CompileCPU="5" CompileMemory="376">
            <RelOp AvgRowSize="15" EstimateCPU="0.00454996" EstimateIO="0.0112613" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Distinct Sort" NodeId="0" Parallel="false" PhysicalOp="Sort" EstimatedTotalSubtreeCost="0.142839">
              <OutputList>
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
              </OutputList>
              <MemoryFractions Input="1" Output="1" />
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRebinds="1" ActualRewinds="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Sort Distinct="true">
                <OrderBy>
                  <OrderByColumn Ascending="true">
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                  </OrderByColumn>
                  <OrderByColumn Ascending="true">
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OrderByColumn>
                </OrderBy>
                <RelOp AvgRowSize="15" EstimateCPU="0.0571741" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Inner Join" NodeId="1" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="0.127028">
                  <OutputList>
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OutputList>
                  <MemoryFractions Input="0" Output="0" />
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                  </RunTimeInformation>
                  <Hash>
                    <DefinedTowns />
                    <HashKeysBuild>
                      <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                    </HashKeysBuild>
                    <HashKeysProbe>
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                    </HashKeysProbe>
                    <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                      <OutputList>
                        <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedTowns>
                          <DefinedTown>
                            <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                          </DefinedTown>
                        </DefinedTowns>
                        <Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
                      </IndexScan>
                    </RelOp>
                    <RelOp AvgRowSize="19" EstimateCPU="0.0357739" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Inner Join" NodeId="3" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.066568">
                      <OutputList>
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <NestedLoops Optimized="false">
                        <OuterReferences>
                          <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                        </OuterReferences>
                        <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="4" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                          <OutputList>
                            <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRows="137" ActualEndOfScans="1" ActualExecutions="1" />
                          </RunTimeInformation>
                          <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                            <DefinedTowns>
                              <DefinedTown>
                                <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                              </DefinedTown>
                            </DefinedTowns>
                            <Object Table="[@County]" Index="[PK__#3E88198__B68F9DF7407061FE]" Alias="[c]" />
                          </IndexScan>
                        </RelOp>
                        <RelOp AvgRowSize="19" EstimateCPU="0.00957118" EstimateIO="0.0179398" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Index Seek" NodeId="5" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.027511" TableCardinality="6291320">
                          <OutputList>
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="137" ActualExecutions="137" />
                          </RunTimeInformation>
                          <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
                            <DefinedTowns>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                              </DefinedTown>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                              </DefinedTown>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                              </DefinedTown>
                            </DefinedTowns>
                            <Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_CountyID_inc_StateID_TownID]" Alias="[t]" IndexKind="NonClustered" />
                            <SeekPredicates>
                              <SeekPredicateNew>
                                <SeekKeys>
                                  <Prefix ScanType="EQ">
                                    <RangeColumns>
                                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                    </RangeColumns>
                                    <RangeExpressions>
                                      <ScalarOperator ScalarString="@County.[CountyID] as [c].[CountyID]">
                                        <Identifier>
                                          <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                                        </Identifier>
                                      </ScalarOperator>
                                    </RangeExpressions>
                                  </Prefix>
                                </SeekKeys>
                              </SeekPredicateNew>
                            </SeekPredicates>
                          </IndexScan>
                        </RelOp>
                      </NestedLoops>
                    </RelOp>
                  </Hash>
                </RelOp>
              </Sort>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
4

2 回答 2

7

虽然我没有深入研究您的查询计划，但我有建议和建议的解释。建议很简单：不要对行数超过两个数量级（100 行）的表使用表变量。可能的解释是，根据 Microsoft 关于表数据类型的文章，它们的性能可能会非常差，并且生成的查询计划可能不准确或效率低下：

最佳实践

不要使用表变量来存储大量数据（超过 100 行）。当表变量包含大量数据时，计划选择可能不是最佳或稳定的。考虑重写此类查询以使用临时表或使用 USE PLAN 查询提示以确保优化器使用适用于您的方案的现有查询计划。

和：

限制和限制

SQL Server 优化器的基于成本的推理模型不支持表变量。因此，当需要基于成本的选择来实现有效的查询计划时，不应使用它们。当需要基于成本的选择时，首选临时表。这通常包括带有连接的查询、并行决策和索引选择选项。

修改表变量的查询不会生成并行查询执行计划。修改非常大的表变量或复杂查询中的表变量时，性能可能会受到影响。在这些情况下，请考虑改用临时表。有关详细信息，请参阅创建表 (Transact-SQL)。读取表变量而不修改它们的查询仍然可以并行化。

...

在实践中，我发现使用表变量会妨碍并行查询计划的生成，并且会损害除最简单、最不复杂的表变量之外的所有表变量的性能。

相反，我发现临时表在几乎所有情况下都具有更高的性能，并且只要您小心地及时清理自己（或使用短期会话以允许服务器清除临时表）然后您获得更好的性能。

至于为什么第二个查询计划成本更低但执行时间更长？很可能有很多很多县（1000 个？10000 个？），并且正在生成的查询计划假设有少量（100 个）。查询优化器认为 @County可能少于 100 行，因此它可能认为结果集将非常具体，因为城镇列表与 <100 条目的列表连接，然后与 <100 的第二列表连接条目的行数可能少于 100 行。

实际上，每个城镇都有一个县，因此连接不是很有选择性，并且使用的连接类型选择很差。在每种情况下，最后一个连接都使用了嵌套循环，但嵌套循环连接对于大型排序数据效率极低。而这很可能是问题的症结所在。

使用临时表而不是表变量重新运行您的查询，我猜您将获得具有非常不同性能特征的非常不同的查询计划。

于 2013-07-11T01:18:31.113 回答
-3

Per Mikael's comment. The first query is faster but has a higher cost because it uses parallelism.

于 2013-07-13T14:52:22.670 回答